With regard to the earlier standpoint of techniques in the present context, reference is made to the subject matter of International Patent Publication PCT/SE96/01448 (corresponding to Swedish Patent Application No. 95 04020-0).
Reference is also made to this publication with regard to the definition of terms used in the present document.
With regard to the properties associated with the present invention, the teachings of the European Patent Publication 0 647 845 are also relevant with regard to the present standpoint of techniques, since it illustrates and describes a gas sensor that includes a gas cell which has a circular, concave mirror surface positioned opposite a concentrically orientated, circular-convex mirror surface, with the light source facing slightly to one side of the mutually coinciding centres of the mirror surfaces.
A gap in the convex mirror surface causes a slightly radially and outwardly directed light bundle with divergent light rays to be reflected alternately in concave and convex mirror sections, whereafter light rays are permitted to exit through a gap provided in the outer concave mirror surface, subsequent to having undergone a predetermined number of reflections and therewith having travelled through a predetermined optical light analyzing path of predetermined distance.
FIG. 6 of this prior publication illustrates an arrangement which is designed to enable the selection of one of two different optical analysis paths within the gas cell with the aid of separate light-ray exiting gaps and through a light-ray entering opening that is common to both analyzing paths.
It can be assumed that the described gas sensor is constructed so that its light receiving means is constructed to sense light rays that are coordinated to form a light bundle reflected in a convex mirror section in the gas cell and therewith a divergent light bundle.
Also forming part of the earlier standpoint of techniques is the arrangement of three partially spherical and opposing concave mirror surfaces designated "White" mirrors.
These mirrors are normally positioned at a relatively large distance from each other and the light source used is normally a laser light source.
A light bundle which contains more or less parallel light rays is reflected several times between the three opposing, spherically concave mirror surfaces, and when the last reflected light bundle is allowed to pass (adjacent the mirror edge) beyond a mirror surface, the light rays in the light bundle are received in a spectral analysis arrangement and evaluated in a known manner.
Also forming part of the earlier standpoint of techniques is the teachings of the Patent Publication U.S. Pat. No. 5,009,493. This publication describes an arrangement for a beam path in a multiple-reflection cell for measuring the absorption of light in a measuring gas. In the measuring arrangement, an entrance aperture is imaged on an exit aperture via an entrance aperture mirror, a field mirror and an exit aperture mirror.
This arrangement is improved in that the astigmatism of the imaging error is considerably reduced. For this purpose, the form of the aperture mirrors is approximately defined by an ellipsoid wherein the focal point spacing is approximately equal to half the distance between the entrance aperture and the exit aperture.